1. Field of the Invention
The present invention relates to a technology for conducting communication between a plurality of communication terminals.
2. Description of the Related Art
With the realization of miniaturization and lightweight of information terminals, it has become common nowadays for people to carry an information terminal. In association with this, study has been vigorously undertaken on the construction of wireless ad hoc network for on-demand communication. In an ad hoc network, base stations and access points are not necessary. It is therefore easy to build a network even in places where no such infrastructure exists. By using an ad hoc network, a plurality of users may enjoy a game together as they join each other, bringing their own portable game devices and communicating wirelessly.
An ad hoc network is built by allowing terminals to communicate with each other using technologies such as IEEE802.11 or Bluetooth. No problem with power is presented when a terminal is capable of receiving power supply from an external power supply on a continuous basis. In the case of portable terminals, however, it is preferable that battery consumption be reduced as much as possible since such terminals are driven by limited battery power. To address this, a power control process in a power saving mode is standardized in communication standards such as IEEE802.11.
FIG. 1 is a timing chart illustrating the operation of stations in a power saving mode standardized in IEEE802.11. First, one of stations A-D sends a beacon signal. A beacon signal is a broadcast signal sent to all stations. A time window called Announcement Traffic Indication Message (ATIM) window is started subsequent to the beacon transmission. In this window, nodes are expected to maintain an active state. In the power saving mode according to the IEEE802.11 standard, each of the stations is allowed to send an ATIM signal during an ATIM window so as to prevent the other stations to go to sleep.
In the example of FIG. 1, station B sends an ATIM signal to station C in unicast communication, and station C returns an ACK signal to station B. Station A and station D do not send or receive an ATIM signal and therefore can go to sleep when the ATIM window expires. Station B and station C cannot go to sleep. When the ATIM window expires, station B sends data to station C. Station C returns an ACK signal to station B after receiving the data. Before a beacon interval as illustrated expires, station A and station D are activated to send or receive a beacon signal. In the subsequent ATIM window, none of the stations sends or receives an ATIM signal. Therefore, all stations go to sleep when the ATIM window is over.
The timing chart of FIG. 1 is an illustration of an extremely simple case intended to explain the power saving mode of the IEEE802.11 standard. In a case where a network comprised of a plurality of portable game devices is built, a larger number of signals are passed back and forth since the status information of the game-devices should be exchanged. For a game application with relatively severe requirement for real-time processing, the status information should be updated frequently. It is thus preferable to send data by multicast communication.
One problem with the power saving mode of the IEEE802.11 standard in multicast communication is that an ATIM window is set despite the fact that an ACK signal is not returned. The standard power saving mode prescribes that each station monitors an ATIM signal from the other stations during the ATIM window, in order to determine a station that can go to sleep. In other words, all stations are forced into an active state in this period even if they do not send status information. If we consider a case of a game application requiring low delay such as a racing game, it often happens that a player keeps an arrow key of a game controlling unit held down and operates cars in the racing game. In this situation, the status information should be sent to the other portable game devices on a continuous basis. During the ATIM window, however, the status information cannot be sent.
In a wireless environment accommodating a plurality of game devices, a problem with signal collision may occur. Improper signal reception could be addressed by retransmission control. For a game application requiring low delay, however, retransmission control may be subject to severe timing constraints.